The present invention relates to a battery charging circuit and a battery charger used to charge a battery.
Rechargeable batteries, such as lithium ion batteries, are often used in electronic devices. When charging such a battery with a charger, charging must be performed within a voltage range specified for the particular battery type. A charge voltage that is lower than the specified range will decrease the battery charge capacity, while a charge voltage that is higher than the specified range will stress the battery and drastically decrease the charge capacity of the battery. Thus, the charger must be able to output a stable voltage. The charge current also has an upper limit. For a lithium ion battery, for example, since the charge voltage range is narrow, the current received from the charger also has an upper limit restriction.
Japanese Laid-Open Patent Publication No. 8-237880 (page 1, FIG. 1) describes a charger in which the input current from a charging power supply is stabilized by first and second current stabilization circuits, which respectively output a large current and a small current. A switching circuit first activates the first current stabilization circuit and charges a battery with the large current. As the battery voltage reaches a predetermined value and approaches a fully charged state, the switching circuit deactivates the first current stabilization circuit and activates the second current stabilization circuit to continue charging with a small current.
Japanese Laid-Open Patent Publication No. 9-233707 (page 1, FIG. 1) describes a charger that can switch modes. In this charger, a switching circuit activates first and second current stabilization circuits and charges a battery with a large current. As the battery voltage reaches a predetermined value, the switching circuit deactivates the second current stabilization circuit to continue charging with just the activated first current activation circuit.
Referring to FIGS. 4A and 4B, a charging circuit for a lithium ion battery that performs charging in two modes, namely, a trickle charge mode and a fast charge mode, is shown. The charging circuit enters a trickle charge mode when starting charging. In the trickle charge mode, charging is performed by supplying current having a fixed and relatively small value I1. This gradually increases the voltage of the battery.
When the voltage reaches a predetermined value V1 (mode switching reference voltage), the charging circuit enters a second charging stage, namely, the fast charge mode. In the fast charge mode, charging is performed by supplying current having a fixed and relatively large value I2.
When the voltage reaches a predetermined value V2 in the fast charge mode, charging is performed continuously while the voltage value is maintained (voltage control mode). In this case, the charge current is gradually decreased. Charging ends when the charge current reaches a fixed current value I3.
A charge circuit 10 that performs such charging will now be discussed with reference to FIG. 3.
The battery charge circuit 10 supplies a charge current to a battery 50, which is connected to an external terminal TM1. The battery charge circuit 10 is supplied with voltage V11 from an external terminal TM2.
The external terminal TM2 is connected to the drain of an NMOS transistor 100. The source of the transistor 100 is connected to a resistor 11. Charge current is supplied to the battery 50 via the source of the transistor 100 from the external terminal TM1.
The gate of the transistor 100 is connected to the gate of another NMOS transistor 101. The drain of the transistor 101 is connected to the external terminal TM2 and supplied with the voltage V11. The transistors 100 and 101 form a current mirror circuit.
The source of the transistor 101 is connected to a switch 13. The switch 13 connects the source terminal of the transistor 101 to either one of external terminals TM3 and TM4. A mode switching circuit 40 is connected to the switch 13. The mode switching circuit 40 measures the voltage between the two terminals of the battery 50 and provides the switch 13 with a switching signal for switching from a trickle charge mode to a fast charge mode. When receiving the switching signal, the switch 13 changes connections from the external terminal TM3 to the external terminal TM4.
The external terminals TM3 and TM4 are also connected to another switch 14. The switch 14 is provided with the switching signal from the mode switching circuit 40 and switches connections in synchronism with the switch 13. In this manner, when provided with the switching signal, the switches 13 and 14 each change connections from external terminal TM3 to external terminal TM4.
The external terminal TM3 is connected to a resistor R1, and the external terminal TM4 is connected to a resistor R2. The resistor R1 is used to determine the value of the current in the trickle charge mode and the resistor R2 is used to determine the value of the current in the fast charge mode. Generally, the charge current in the fast charge mode is set to be about twenty times greater than the charge current in the trickle charge mode. Accordingly, the resistance values of the two resistors R1 and R2 are set to have a difference of about twenty. Further, parasitic capacitances C1 and C2 are added to the resistors R1 and R2.
The switch 14 has an output terminal connected to a non-inverting input terminal of an error amplifier 121. The error amplifier 121 has an inverting input terminal supplied with voltage V12, which serves as a current restriction reference voltage. The voltage V12 is used as a reference for limiting or restricting current. The output of the error amplifier 121 is provided to a mixer 120. The mixer 120 restricts the gate voltage supplied to the transistors 100 and 101 when the input voltage exceeds the reference value in the error amplifier 121 or an error amplifier 122.
The mixer 120 is also provided with the output of the error amplifier 122. The error amplifier 122 has an inverting input terminal supplied with voltage V13, which serves as a voltage restriction reference voltage, and a non-inverting input terminal that is supplied with a divisional voltage produced by the resistor 11. The voltage V13 also is used as a reference for restricting voltage.
The output of the mixer 120 is provided to the gates of the transistors 100 and 101. When the divisional voltage produced by the resistor 11 is less than the voltage V13 in the error amplifier 122, the mixer 120 controls the gate voltage of the transistors 100 and 101 in accordance with the output from the error amplifier 121 so as to supply a constant charge current. When the divisional voltage produced by the resistor 11 exceeds the voltage V13, the mixer 120 shifts to the voltage control mode and controls the gate terminal voltage to maintain a constant charge voltage.
In the battery charge circuit 10, the connections of the resistors R1 and R2 are changed to switch from the trickle charge mode to the fast charge mode. This produces a phase lag due to a CR time constant. The time constant of the trickle charge mode is C1·R1, and the time constant of the fast charge mode is C2·R2. In this case, the difference between the two resistance values results in a large difference in the phase lag. The time constants are included in a loop formed by the switch 14, the error amplifier 121, the mixer 120, the transistor 101, and the switch 13. This makes it difficult to perform phase compensation. Thus, oscillation or the like destabilizes the operation of the charge circuit 10. In particular, when designing the battery charge circuit 10 so as to prevent oscillation in the fast charge mode, a phase margin for the trickle charge mode may become small and may produce a large oscillation as shown in FIGS. 4A and 4B.